This invention relates generally to ion sources and in particular to field desorption ion sources.
Ion sources, in particular proton sources, with rather homogeneous energy in the emitted beam, that is, beams with good monochromaticity, are needed in microscopes in which ions are used instead of the usual electrons for the purpose of lowering the limit to the resolution set by diffraction effects of the particle beam.
In the past, attempt to use ion beams in high resolution electron microscopes, either conventional transmission or scanning microscopes, were not successful because the available ion sources were not adequate with respect to "brightness". That is, they did not have sufficient target current density per steradian to produce an adequate image.
In addition, the ion sources of the prior art had too broad an energy spread to permit sharp focusing. Where the lack of "brightness" of the prior art devices limited the signal in the image that was to be detected, the broad energy spread of the ions additionally limited the resolution, i.e. the sharpness of the image due to the chromatic aberration of the objective lens.
It is especially important in ions scanning transmission microscopy to have a monchromatic ion source since the image contrast is preferably obtained in this operation by energy analysis of the ions after their passage through the target rather than by selection according to the scattering angle of the beam by means of an aperture.
In the more recently developed instrument called the ion microprobe, secondary ions are used to generate the signal. The limitation here is the signal strength at high resolution. The highest resolution attainable at the moment corresponds to a 1 micron spot diameter of the primary ion beam. To go to a 0.1 micron diameter spot and retain the total spot current, requires a 100 times larger brightness of the primary ion source. For the increased source brightness, the ion source described herein is well suited.
There exist in the prior art, other field-ionization ion sources in which the brightness is adequate, e.g. 10.sup.5 A cm.sup.-.sup.2 sr.sup.-.sup. 1, however, the energy spread in the beam cannot be reduced below 1 eV (H. Heil and R. Guckenberger, Proc. Symp. Ion Sources and Formation of Ion Beams, p. 183, BNL 50310, 1971).
Because field ionization occurs within a narrow range of distances above the electrode tip surface, even though the range may be less than 1 Angstrom in thickness, the change of electrical potential across that distance still amounts to more than 1 volt, and the potential at the position of the origin of the ions and hence the initial energy vary by more than 1 Volt.
High brightness may also be obtained with ion sources of the duoplasmatron type, however, to operate that type of ion source high magnetic fields and large amounts of energy are required. Consequently, the electrodes are operated at very high temperatures resulting in rapid wear and deterioration of the electrode material leading to electrical and mechanical breakdown.
In addition, for both field ionization and duoplasmatron ion sources, a pressure of several mTorr of the gas that is to be ionized must be provided in the ionization space or chamber and the ion beam must be extracted out of the ionization space or chamber through an aperture into the high vacuum region. The escape of unionized gas through this same aperture is oftentimes a nuisance, especially if an ultra-high vacuum is required on the specimen space for work on atomically clean surfaces.
Another use of the ion source of the present invention is in the area of gas flow through metals.
The conventional method of measuring the gas flow is to provide at the vacuum side of the premeation apparatus a mass spectrometer or residual gas analyser. These instruments can detect fluxes as small as 10.sup.12 molecules per second. The field desorption apparatus of the present invention can detect gas fluxes of single ions at rates of 10 counts per second.